I attempted to use our dishwasher to clean a greasy, soot-covered barbeque pan (a bad idea, by the way). The dishwasher stopped, with a blocked outflow indication. I opened it, used a wet vac to drain it, and cleaned out the internal filters, sump pump, and anti-backflow valve. This got the water circulating inside again, but it still wouldn't drain.
Turns out, enough crud had pumped up to clog the air gap. (The air gap is an anti-siphon device, often a small chrome cylinder by the sink.) Once I opened that up and cleaned it, everything was flowing like normal again.
Here's some more information about air gaps:
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Larry Lang